The present invention relates generally to polyimide compositions and more particularly to metal clad laminates made therefrom wherein the laminates have improved peel strength.
Polyimides constitute a class of valuable polymers characterized by high thermal stability, inert character, insolubility in strong solvents, and high glass transition temperatures, among other properties. Polyimide precursors are polyamic acids, which are then imidized by either chemical or thermal processing to form a polyimide.
Polyimide films are found in a wide variety of applications in the flexible circuit industry. These films are used primarily as the base dielectric material in the construction of the flexible circuit. In the manufacture of circuit laminates, the polyimide films are generally clad with a metal layer, usually copper. As used herein the term xe2x80x9cmetal layerxe2x80x9d means a layer made from a single metal such as copper, tin, chromium, nickel, silver or gold or a metal alloy. The metal layer may be in the form of a prefabricated metal foil, which is subsequently bonded to the surface of the polyimide film substrate. Bonding is accomplished by well-known means, such as the use of numerous types of adhesives. It is also known to cast an imidizable polyamic acid solution directly onto a metal foil, thereafter imidize the polyamic acid, and drive off the solvents which ultimately accomplishes the bonding.
Another method of bonding a metal layer to a polyimide film involves sputtering or vapor depositing metal onto the surface of the film. This step is typically followed by an electroplating or electroless-plating step, which increases the metal layer to the desired thickness. Those skilled in the art will appreciate that sputtering and plating in general may be used to deposit and bond, a plurality of different metallic foils to the surface of the polyimide substrate. For example, it is known to first sputter a chromium layer, followed by a copper layer, followed by copper electroplating to produce a copper foil laminated polyimide film. Thus, the term xe2x80x9cmetal layerxe2x80x9d as used herein includes a single layer of a single metal, or may include an alloy, and may include multiple layers of differing metals and alloys.
Circuitry is fabricated by known etching techniques applied to the polyimide/metal laminate.
It is believed that a common location for failure in the circuit laminates is in the outer surface of the polyimide film. It is theorized that a weak boundary layer may exist at the surface of the polyimide, which ultimately becomes the xe2x80x98weak linkxe2x80x99 in the laminate construction. It is believed that the failures may occur at a depth of approximately 10 nm into the polyimide film.
It has been known in the art of polyimide film processing to use tin additives in the polyimide composition to improve the peel strength of metal layer clad polyimide films. However, these additives have been found to cause a color change in the film, which is undesirable in the circuitry industry. Thus, a need exists for a technique that improves the peel strength of the metal clad polyimide film laminate without changing the color of the polyimide film.
In accordance with the present invention there is provided a polyimide composition which, when made in film form and clad with a metal layer, exhibits improved peel strength between the film and metal layer. The polyimide composition of the invention comprises the reaction product of components comprising:
(a) a polyamic acid, said polyamic acid being dissolved in a solvent so as to form a solution;
said polyamic acid having a minimum gel-film formation temperature and a minimum green film formation temperature associated therewith;
(b) an esterified polyamic acid oligomer having from two to twenty repeating units;
said oligomer having at least two crosslinkable groups selected from the group consisting of carbonyl, cyano, hydroxy, alkyne, maleimide, norbornene and sulfonyl groups;
said oligomer having an imidization temperature associated therewith; and
said imidization temperature of said oligomer being greater than the minimum gel-film formation temperature or the minimum green film formation temperature; and
said esterified polyamic acid oligomer being soluble in said polyamic acid solution and present in an amount of 0.5 to 10 weight percent of the combined weights of components (a) and (b).
The peel strength of laminates made by cladding the polyimide film of the present invention with a copper metal layer have been determined using the IPC Peel Strength, Flexible Printed Wiring Materials method 2.4.9, Revision C, Method B when an acrylic adhesive is used. Peel strength of a laminate, when sputtering and electroplating methods are used in the fabrication, is measured by Method 2.4.9A of IPC-TM-650. Both test results are reported in pounds per linear inch, pli (N/cm). The test results reported herein reveal that the laminates, made using either an adhesive or sputtering and electroplating, exhibit peel strengths of at least 8 pli (14 N/cm). Peel strengths as high as 12-14 pli (21-24 N/cm) have been observed. The present invention also has the desired feature that no color change in the polyimide composition, such as the color change associated with tin additives, is observed in the polyimide film.
The polyimide composition of the invention comprises the reaction product of a polyamic acid (a) and an esterified polyamic acid oligomer (b).
As is known in the art, the polyamic acid (a) is a reaction product of one or more dianhydride monomers and one or more diamine monomers. The polyamic acid (a) is capable of imidization, either by chemical or thermal conversion, thereby forming a polyimide.
In accordance with the present invention, the polyamic acid is dissolved in a solvent so as to form a polyamic acid solution.
When operating a chemical conversion process, the polyamic acid solution (a) has a xe2x80x9cminimum gel-film formation temperaturexe2x80x9d associated therewith. As used herein the term xe2x80x9cminimum gel-film formation temperaturexe2x80x9d means that temperature at which imidization of the polyamic acid occurs, in a chemical conversion process, to such an extent that a self-supporting gel-film is formed within twenty minutes. The minimum gel-film formation temperature may be as low as 15xc2x0 C. It is understood by those skilled in the art however, that gel-film formation temperatures well in excess of the minimum are preferably employed so that a self-supporting gel-film is formed in a much shorter time. In a continuous film casting operation for example, the formation of the self-supporting gel-film preferably occurs in less than two minutes. This corresponds to gel-film formation temperatures between about 60xc2x0 and 125xc2x0 C. During the gel-film formation step, solids content of the gel film is typically about 20 weight percent.
When utilizing a thermal conversion process, the polyamic acid solution (a) has a xe2x80x9cminimum green film formation temperaturexe2x80x9d associated therewith. As used herein the term xe2x80x9cminimum green film formation temperaturexe2x80x9d means that temperature at which solvent loss and imidization of the polyamic acid occurs, in a thermal conversion process, to such an extent that a self-supporting green film is formed in sixty minutes or less. The minimum green film formation temperature may be as low as 50xc2x0 C. Higher green film formation temperatures are employed to form self-supporting films in shorter times. As a practical matter however, green film formation temperatures in excess of 200xc2x0 C. are not generally used because poor film quality results. The green film has a solids content that is typically about 75 weight percent and the level of imidization is generally only 25 to 30% of full imidization.
The esterified polyamic acid oligomer (b) is soluble in the polyamic acid solution (a) and has an imidization temperature associated therewith. As used herein in connection with the esterified polyamic acid oligomer (b) the term xe2x80x9cimidization temperaturexe2x80x9d means that temperature at which substantial imidization occurs at the ester sites, yielding an alcohol byproduct. Preferably, the imidization temperature of the esterified polyamic acid oligomer (b) is at least 140xc2x0 C., regardless whether chemical or thermal conversion is employed. This is in stark contrast to simple polyamic acids where imidization can occur even at room temperature with chemical conversion. Thus, the minimum gel-film and/or green film formation temperature the polyamic acid solution (a) and the actual gel-film and/or green film formation temperature employed, is less than the imidization temperature of the esterified polyamic acid oligomer (b).
The esterified polyamic acid oligomer (b) has a least two crosslinkable groups selected from the group consisting of carbonyl, cyano, hydroxy, alkyne, maleimide, norbornene, and sulfonyl groups. While not intending to be bound by any particular theory, it is believed that during the drying and imidization steps in polyimide film manufacture, the oligomer (b) may diffuse toward the surface of the film and impart that portion of the film with greater strength as a result of forming cross-linked bonds.
The esterified polyamic acid oligomer (b) can have from 2 to 20 repeating units, and preferably from 2 to 7 repeating units. Less than two repeating units does not comprise an oligomer. At more than about twenty repeating units, the oligomer begins to take on the characteristics of a polymer. An esterified polyamic acid oligomer of no greater than 20 units ensures that it can readily diffuse to the surface of the polyamic acid (a), in either a gel-film or green film state, prior to full imidization of the sheet.
In order to effectively impart improved peel resistance to laminates made by cladding the polyimide of the invention with a metal layer, the esterified polyamic acid oligomer is present in an amount of 0.5 to 10 weight percent and preferably in an amount of 1.0 to 3.0 weight percent based upon the combined dry weights of the polyamic acid and esterified polyamic acid oligomer.
Polyamic Acid Solution
The polyamic acid solution (a) comprises a polyamic acid prepared from a tetracarboxylic acid or dianhydride component, and a diamine component. This is done in the presence of a polar, aprotic solvent. Preferably, the polyamic acid has a minimum gel-film formation temperature or a minimum green film formation temperature below 140xc2x0 C.
Polyamic Acid Monomer Selection
The polyamic acid used in the present invention is prepared by copolymerizing substantially equimolar amounts of an organic tetracarboxylic acid (or its dianhydride) and a diamine. Aromatic tetracarboxylic acids and diamines may be employed. The aromatic tetracarboxylic acid component may include biphenyltetracarboxylic acid or its functional derivative, pyromellitic acid, or other functional derivatives, or combinations of both. Some suitable examples of the aromatic tetracarboxylic acid component include pyromellitic acid and its dianhydride; 3,4,9,10-perylene tetracarboxylic dianhydride; naphthalene-2,3,6,7-tetracarboxylic dianhydride; naphthalene-1,4,5,8-tetracarboxylic dianhydride; 4,4xe2x80x2-oxydiphthalic dianhydride; 3,3xe2x80x2,4,4xe2x80x2-biphenylsulfone tetracarboxylic dianhydride; 2,3,2xe2x80x2,3xe2x80x2-benzophenonetetracarboxylic dianhydride; 3,3xe2x80x2,4,4xe2x80x2-benzophenone tetracarboxylic dianhydride; bis(3,4-dicarboxyphenyl)sulfide dianhydride; bis(3,4-dicarboxyphenyl)methane dianhydride; 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride; 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane; 2,2xe2x80x2,3,3xe2x80x2-biphenyltetracarboxylic dianhydride; 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic dianhydride; 2,6-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride; 2,7-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride; 2,3,6,7-tetrachloronaphthalene-1,4,5,8-tetracarboxylic dianhydride; phenanthrene-,8,9,10-tetracarboxylic dianhydride; pyrazine-2,3,5,6-tetracarboxylic dianhydride; benzene-1,2,3,4-tetracarboxylic dianhydride; and thiophene-2,3,4,5-tetracarboxylic dianhydride.
Some suitable organic diamines include meta-phenylenediamine; para-phenylenediamine; 1,2-diaminobenzene; 4,4xe2x80x2-diaminodiphenylmethane; 2,2-bis(4-aminophenyl)propane; 4,4xe2x80x2diaminodiphenyl propane; 4,4xe2x80x2-diaminodiphenyl sulfide; 4,4xe2x80x2-diaminodiphenyl sulfone; 3,3xe2x80x2-diaminodiphenyl sulfone; 3,4xe2x80x2diaminodiphenyl ether; 4,4xe2x80x2-diaminodiphenyl ether; 2,6-diaminopyridine; bis(3-aminophenyl)diethyl silane; 4,4xe2x80x2diaminodiphenyl diethyl silane; benzidine; 3,3xe2x80x2-dichlorobenzidine; 3,3xe2x80x2-dimethoxybenzidine; 4,4xe2x80x2-diaminobenzophenone; N,N-bis(4-aminophenyl)-n-butylamine; N,N-bis(4-aminophenyl)methylamine; 1,5-diaminonaphthalene; 3,3xe2x80x2-dimethyl-4,4xe2x80x2-diaminobiphenyl; 4-aminophenyl-3-aminobenzoate; N,N-bis(4-aminophenyl)aniline; 2,4-bis(beta-amino-t-butyl)toluene; bis(p-beta-amino-t-butylphenyl)ether; p-bis-2-(2-methyl-4-aminopentyl)benzene; p-bis(1,1-dimethyl-5-aminopentyl)benzene; 1,3-bis(4-aminophenoxy)benzene; m-xylylenediamine; p-xylylenediamine; 4,4xe2x80x2-diaminodiphenyl ether phosphine oxide; 4,4xe2x80x2-diaminodiphenyl N-methyl amine; and 4,4xe2x80x2-diaminodiphenyl N-phenyl amine.
The preparation of polyimides and polyamic acids is more fully described in U.S. Pat. Nos. 3,179,614; 3,179,630; and 3,179,634 the disclosures of which are incorporated herein by reference.
The most preferred polyamic acid for the present invention is prepared from dianydride monomers comprising pyromellittic dianhydride, alone or in combination with up to 70 mole % biphenyldianhydride, and preferably 20-70 mole % biphenyldianhydride. Preferred diamines comprise oxidianiline, used alone or in combination with, up to 90 mole % para-phenylene diamine, and preferably 10-90 mole % para-phenylene diamine. Copolymerization of the dianhydride and diamine monomers is carried out in an inert solvent at temperatures not higher than 140xc2x0 C., preferably not higher than 90xc2x0 C. for about one minute to several days. The components can be added either neat, as a mixture, or as solutions to the organic solvent, or the organic solvent may be added to the components.
It is not required that the tetracarboxylic acid (or its dianhydride) and the diamine components be used in absolutely equimolar amounts. In order to adjust the molecular weight, the molar ratio of tetracarboxylic acid to diamine component can range from 0.90 to 1.10.
The polyamic acid solution prepared as described above may contain polyamic acid polymer in an amount from approximately 5% to 40% and preferably 10% to 25% by weight in the solvent.
Organic Solvent
The organic solvent should dissolve the monomeric components and the polyamic acid formed therefrom. The solvent must be substantially unreactive with all of the monomeric components and with the polyamic acid. Preferred solvents include normally liquid N,N-dialkylcarboxylamides such as N,N-diethyl-formamide and N,N-diethylacetamide. Other solvents that may be used are dimethylsulfoxide, N-methylpyrrolidone, N-cyclohexyl-2-pyrrolidone, tetramethyl urea, dimethylsulfone, hexamethylphosphoramide, tetramethylenesulfone, diglyme, pyridine and the like. The solvents can be used alone or in combination with themselves or in combination with poor solvents such as benzene, benzonitrile and dioxane.
Esterified Polyamic Acid Oligomer
The esterified polyamic acid oligomer (b) is soluble in the polyamic acid solution (a) and has an imidization temperature which is greater than the minimum green film or minimum gel-film formation temperature of the polyamic acid polymer. The oligomer includes at least two crosslinkable groups (groups that are typically used specifically in the thermosetting polyimide industry) selected from the group consisting of carbonyl, cyano, hydroxy, alkyne, maleimide, norbornene, and sulfonyl groups. The crosslinkable groups may have their origin in the dianhydride or diamine components or in an endcapping agent, as is explained below.
Suitable dianhydrides for the preparation of the esterified polyamic acid oligomer (b) include pyromellitic dianhydride, 2,2xe2x80x2,3,3xe2x80x2-biphenyltetracarboxylic dianhydride; 3,3xe2x80x2,4,4xe2x80x2-biphenyl tetracarboxylic dianhydride; 4,4xe2x80x2-oxydiphthalic anhydride; 2,3,2xe2x80x2,3xe2x80x2-benzophenonetetracarboxylic dianhydride; 3,3xe2x80x2,4,4xe2x80x2-diphenylsulfone tetracarboxylic dianhydride; bis(3,4-dicarboxyphenyl)methane dianhydride; 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride; and the most preferred being 3,3xe2x80x24,4xe2x80x2-benzophenone tetracarboxylic dianhydride.
Suitable diamines for the preparation of the esterified polyamic acid oligomer (b) include m-phenylene diamine; para-phenylene diamine; diaminobenzophenone, and 4,4xe2x80x2-oxydianline; 3,4xe2x80x2-oxydianiline; 4,4xe2x80x2-diaminodiphenylmethane; 2,2-bis(4-aminophenyl)propane; 4,4xe2x80x2diaminodiphenyl propane; 4,4xe2x80x2-diaminodiphenyl sulfone; 3,3xe2x80x2-diaminodiphenyl sulfone; bis(3-aminophenyl)diethyl silane; 4,4xe2x80x2diaminodiphenyl diethyl silane; benzidine; 1,3-bis(4-aminophenoxy)benzene; p-diaminodiphenyl acetylene; 3,3xe2x80x2-dihydroxy-4,4xe2x80x2-diamino-biphenyl; 3,3xe2x80x2-diamino-4,4xe2x80x2-dihydroxy-biphenyl; 1,3-bis(3-aminophenoxy)-5-cyano-benzene; the most preferred being m-phenylene diamine.
The molecular weight of the esterified polyamic acid oligomer (b) can be controlled in two ways. In the first way, alcohols or amine compounds may be added to the dianhydride to form a monoanhydride. For example, in the case of an alcohol being added to the dianhydride, the monoanhydride formed will have an acid group and an ester group, or may have two ester groups. Generally, the amount of alcohol or amine compound required is about 10-70 mole %, preferably 50 mole %, based on the total number of anhydride groups. Ultimately, it is the formation of an ester group on the anhydride that will later prevent polymerization with a diamine. However, this method is not preferred because final chain length of the oligomer is difficult to control.
The preferable method of oligomer chain length control is to add an endcapping compound such as nadic anhydride, maleic anhydride, phenyl ethynyl phthalic anhydride, and most preferably phthalic anhydride, to the diamine of the oligomer. The amount of endcapping compound should be 10-70 mole %, preferably 50 mole %, based on the total number of amine groups. By endcapping the diamine prior to reaction with the dianhydride, final oligomer chain length is controlled. After the use of either method described above, substantially all the anhydride and amine species are reacted together to form a polyamic acid oligomer.
To enable esterification of the polyamic acid oligomer a catalyst, like trifluoroacetic anhydride, is added to xe2x80x9cring closexe2x80x9d the polyamic acid oligomer precursor to form an oligo (iso-imide). After forming the oligo (iso-imide), a stoichiometric excess of an alcohol with respect to the number of iso-imide groups, is added to the oligo (iso-imide) to obtain the esterified polyamic acid oligomer (b) of the present invention.
Ethanol is the preferred alcohol used for esterification of the polyamic acid oligomer precursor. However, it is believed that any aliphatic alcohol, which has less than about ten carbon atoms, can be used. One practicing this invention should note that when an excess amount of alcohol is added, which should theoretically provide for full esterification, a considerable number of amic acid groups remain present. The present inventors have found that in order to obtain the improved peel strengths attributable to the present invention, a minimum of at least 10% of the iso-imide groups should be esterified. Esterification from 30% to 40% is preferred.
Preferably, the esterified polyamic acid oligomer (b) is represented by the following formula: 
wherein n is equal to 2 to 20;
wherein each R1 is independently selected from the group consisting of: 
wherein each R2 is independently selected from the group consisting of: 
xe2x80x83with the proviso that no more than 90% of said R2 groups are xe2x80x94OH groups;
wherein each R3 is independently selected from the group consisting of: 
wherein each R4 is independently selected from the group consisting of: 
with the proviso that the oligomer has at least two crosslinkable groups selected from the group consisting of carbonyl, cyano, hydroxy, alkyne, maleimide, norbornene and sulfonyl groups.
Most preferably, the esterified polyamic acid oligomer (b) is represented by the following formula: 
wherein n is 2 to 7, each R2 is independently selected from the group consisting of ethoxy and hydroxy groups, with the proviso that no more than 90% of said R2 groups are hydroxy groups.
In accordance with the present invention, the amount of the esterified polyamic acid oligomer (b) may vary from approximately 0.5 to 10% by weight based on the combined dry weights of the polyamic acid and esterified polyamic acid oligomer. Preferably, the amount of esterified polyamic acid oligomer (b) is from 1.0 to 3.0% by weight based on the combined dry weights of the polyamic acid and esterified polyamic acid oligomer.
Process of Manufacturing the Film
The polyamic acid (a) and the esterified polyamic acid oligomer (b) may be converted using either a chemical or thermal conversion film process.
Chemical Conversion
In the chemical conversion process, the polyamic acid solution (a) (including the esterified polyamic acid oligomer (b)) is either immersed in, or mixed with conversion chemicals. Polyamic acid conversion chemicals are typically anhydride dehydrating materials and tertiary amine catalysts. The preferred anhydride dehydrating material is acetic anhydride and is often used in molar excess of the amount of amide acid groups in the polyamic acid. Typically about 2 to 2.4 moles of anhydride dehydrating material per repeating unit of polyamic acid is used. A comparable amount of tertiary amine catalyst is often used.
Other operable lower fatty acid anhydrides, which may be used in place of acetic anhydride, include propionic anhydride, butyric anhydride, valeric anhydride and mixtures thereof. These anhydride mixtures can also be combined with mixtures of aromatic monocarboxylic acids for example, benzoic acid or naphthoic acid, or with mixtures of anhydrides of carbonic and formic acids, as well as aliphatic ketenes (ketene and dimethyl ketene). Ketenes may be regarded as anhydrides of carboxylic acids derived from drastic dehydration of the acids.
The preferred tertiary amine catalysts are pyridine and beta picoline. They are used in varying amounts up to several moles per mole of anhydride dehydrating material. Other tertiary amines having approximately the same activity as the preferred pyridine and beta-picoline and may also be used. These include alpha-picoline; 3,4-lutidine; 3,5-lutidine; 4-methyl pyridine; 4-isopropyl pyridine; N,N-dimethylbenzyl amine; isoquinoline; 4-benzyl pyridine, N,N-dimethyldodecyl amine and triethyl amine.
In the chemical conversion process, the polyamic acid solution (a) (including the esterified polyamic acid oligomer (b) and conversion chemicals) is cast on a surface such as a metal drum or belt and heated to a temperature at or above the minimum gel-film formation temperature of the polyamic acid (a), but below the imidization temperature of the esterified polyamic acid oligomer (b). The self-supporting gel-film is stripped off the surface and transported to a tenter oven to complete imidization under high temperature processing. In the tenter oven, the film is heated to temperatures in excess of the imidization temperature of the esterified polyamic acid oligomer (b). Thus, the film is dried of remaining solvents, becomes fully imidized and the crosslinking of the oligomer to the rest of the polyimide film is complete.
Another method of chemically converting the polyamic acid solution (a) (including the esterified polyamic acid oligomer (b)) is to extrude the acid solution into a bath of conversion chemicals. The bath contains anhydride and tertiary amine components, and may or may not contain a diluting solvent. The extruded acid solution is then subjected to a temperature at or above the minimum gel-film formation temperature, but not to a temperature that is greater than the imidization temperature of the esterifed polyamic acid oligomer (b). Again, a self-supporting gel-film is formed.
Next, the gel-film is heated in a tenter oven to a temperature in excess of the imidization temperature of the esterified polyamic acid oligomer (b). The volatiles are removed, full imidization is achieved, and oligomer crosslinking is complete. Because the gel-film in chemical conversion process has a high liquid content, it must be restrained during the heating step to avoid undesirable shrinkage.
During continuous production of a film, the film is clipped or pinned at the edges for restraint. As part of the manufacturing process, the film can be stretched in either the machine direction, or the transverse direction, during final heating. Final film temperatures between 400xc2x0 C. and 500xc2x0 C. are required to achieve optimum film properties.
Thermal Conversion
In a thermal conversion process the polyamic acid solution (a) (including the esterified polyamic acid oligomer (b)) is cast onto a surface and heated to a temperature at or above the minimum green film formation temperature. The highest casting temperature employed should be lower than the imidization temperature of the esterified polyamic acid oligomer (b).
The self-supporting green film is stripped from the casting surface and transported to a tenter oven for high temperature processing and full imidization. In the tenter oven, the film is heated to temperatures in excess of the imidization temperature of the esterified polyamic acid oligomer (b), so that the film is dried of remaining solvents, is fully imidized, and oligomer crosslinking is complete.
The advantageous properties of the present invention can be observed by referencing the following examples, which illustrate but do not limit, this invention. All parts and percentages are by weight unless otherwise indicated. In the discussion above and in the examples RH means relative humidity.